Method and apparatus for making an offset printing plate

ABSTRACT

The present invention concerns a plate making method of forming an image on a heat sensitive blank plate by a multichannel method using a plate making apparatus of an outer surface cylinder scanning type. A blank plate ( 400 ) is secured to the outer circumferential surface of a hollow cylinder ( 131 ). A laser beam ( 800 ) is irradiated from an optical head ( 150 ) onto the blank plate ( 400 ). The laser beam ( 800 ) is a beam group consisting of a plurality of infrared laser beams arranged in a line.

TECHNICAL FIELD

The present invention concerns a method of making a heat sensitive typeoffset printing plate and a manufacturing apparatus capable of easilypracticing the method.

BACKGROUND ART

An apparatus adapted to irradiate a laser beam selectively on asensitive material based on an image recording signal thereby forming animage has been known so far as a film plotter or an image setter. Forexample, Japanese Patent Unexamined Publication No. 60-203071 disclosesa laser plate making apparatus of forming an image by a plurality oflaser beams.

On the other hand, along with popularization of computers or developmentof network techniques typically represented by internets in recentyears, a CTP (computer to plate) system of directly making a plate foroffset printing from digital image data edited on a computer not by wayof a negative film or a positive film has been enabled. Then, the CTPsystem has attracted attention as a substitute for a PS (pre-sensitized)plate system using a film, which is predominant in offset printing atpresent.

A system already put to practical use as a plate making system of anoffset printing plate used for the CTP system is a print making systemusing photosensitive materials such as an OPC (organicphoto-semiconductor), a silver salt, a hybrid material of a silver saltand a photopolymer and a highly sensitive photopolymer as a blank plate.However, it is necessary for the print making system described abovethat the blank plate has to be handled in a dark room like that existentPS print systems. Further, the plate making systems described aboverequires a developing step after the image drawing step to the blankplate like that the existent PS plate system and, therefore, involves aproblem for discarding treatment of a liquid developer or the like.

On the contrary, a CTP plate making system using a heat sensitive typeblank plate having a response region in an infrared region, the blankplate can be handled in a light room. Further, a great amount of heatenergy is charged in an image forming step by a laser beam in thissystem, thereby an image is formed by thermally converting a portion, towhich an image is formed, of a heat sensitive layer from hydrophilic tooleophilic property, so it requires no developing step. Accordingly,such a heat sensitive type CTP system has been noted as a CTP system inthe next generation.

Generally, plate making apparatus used for the CTP system are broadlyclassified, depending on the difference of scanning system, into threetypes of an outer surface cylinder scanning system, an inner surfacecylinder scanning system and a planer scanning system. A laser platemaking apparatus of the outer surface scanning system is disclosed, forexample, in Japanese Patent Examined Publication No. 51-46138.

As the plate making apparatus used for photosensitive blank plates, aplate making apparatus of an inner surface cylinder scanning system ofsecuring a blank plate to a cylinder inner surface and scanning a laserbeam by a rotational end face mirror has been utilized generally, sincethis can conduct a high speed scanning and also easily cope withdifferent sizes of blank plates. However, the inner surface cylinderscanning type plate making apparatus is not suitable as a plate makingapparatus for heat sensitive type blank plates with the reason describedbelow.

That is, since a heat sensitive type blank plate generally has asensitivity lower by about three digits compared with a photosensitiveblank plate, when the inner surface cylinder scanning system is adopted,it requires an expensive solid laser of excellent beam characteristics,for example, an Nd-YAG laser capable of providing an extremely highoutput energy and having a long focal distance. However, since thesensitive wavelength region of usable blank plates is restricted to 1064nm as an emitting wavelength of an Nd-YAG laser, the degree of freedomfor the design of the blank plate is lowered in the inner surfacecylinder scanning type plate making apparatus using the Nd-YAG laser asan image forming laser.

On the contrary, a semiconductor laser having a central light emissionwavelength region near 750-880 nm is inexpensive compared with theNd-YAG laser. Accordingly, use of the semiconductor layer for the imageforming laser is preferred in order to reduce the apparatus cost of theheat sensitive type CTP system. However, since no long focal distancecan be available in the semiconductor laser in view of beamcharacteristics, it is difficult to adopt the inner surface cylinderscanning system in the plate making apparatus using the semiconductorlaser.

Accordingly, in the plate making apparatus using the semiconductorlaser, an outer surface cylinder scanning system, that is, a system ofwinding a blank plate around the outer surface of a cylinder, andirradiating a laser beam to the blank plate from an optical headdisposed near the cylinder outer surface is adopted. The plate makingapparatus of this type is adapted, for example, such that a laser beamirradiated from a semiconductor laser is transmitted through an opticalfiber and introduced to the optical system of an optical head disposednear the cylinder outer surface and a laser beam focused by an objectivelens at the top end of the optical system to the blank plate at thecylinder outer surface.

In the plate making apparatus of the outer surface cylinder scanningsystem described above, with an aim of increasing the plate makingspeed, an image is formed by a so-called multi-channel system of using aplurality of semiconductor lasers to increase the number of scanninglines per one rotation of the cylinder.

Then, in a general multi channel system plate making apparatus, aplurality of laser beams are arranged each at an equal interval in lineand the beams are formed into a group of beams parallel with each otherand the beam group is introduced to a set of optical systems.

However, when an image is formed by a plurality of infrared laser beamsarranged in line, heat of infrared rays is absorbed in the heatsensitive layer, as well as a great amount of heat is also generated bychemical reaction in the heat sensitive layer. Then, the heat diffusesto the periphery by heat conduction while elevating the temperature ofthe blank plate. Accordingly, the temperature of the image area of theblank plate formed with an image by a beam situated at the center of theline is higher compared with that in the image region of the blank plateformed with an image by a beam situated on the end of the line.

As described above, when an image is formed by a plurality of infraredlaser beams disposed in line to the heat sensitive blank plate to beformed with images by thermal reaction, since a temperature distributionis caused to the blank plate upon image formation, it is difficult toform an image uniformly over the entire image formation region. That is,there is a room for the improvement of the image quality of the printingplate obtained by this method.

On the other hand, in the process color printing, a color image isseparated into that of four colors, namely, Y (yellow), M (magenta), C(cyan) and K (black), and a plate for each color is made, and each ofimages is printed with an ink of a corresponding color by using the fourplates. Then, color printed matters of good quality can be obtained byoverlapping images printed by the four plates with inks of differentcolors on an exact position of paper. Positional alignment for each ofthe plates in a printing machine is carried out by disposing one side asa reference to each of the plates and aligning the sides to each other.Accordingly, also in the plate making, an image has to be formed at anaccurate position with the side being as a reference.

However, the plate making apparatus of the outer surface cylinderscanning system in the prior art still has a room for the improvement inview of convenient and accurate positioning upon attaching the blankplate to the cylinder.

Further, Japanese Patent Unexamined Publication No. 7-1849 discloses amaterial for forming a heat sensitive layer constituting a heatsensitive type blank plate, which contains microcapsules containing anoleophilic ingredient in the inside and destroyed by heat, hydrophilicbinder polymer having functional groups capable of three dimensionalcross linking and functional groups capable of reacting with theoleophilic ingredient, and photoreaction initiator for initiating threedimensional cross linking reaction of the hydrophilic binder polymer.However, the printing plate made by the existent method using the heatsensitive blank plate having the material as a heat sensitive layer isinsufficient in the printing resistance for an image area and stillleaves a room for improvement in the printing quality of the obtainedprinting plate.

The present invention has been accomplished taking notice on theproblems in the prior art described above, and it is a subject thereofto remarkably improve the quality of the image to be formed and theprinting quality in the image area, upon making the heat sensitive blankplate into a printing plate by the outer surface cylinder scanningsystem plate making apparatus and, further, enable to accuratelyposition images of four colors by a convenient method in a short periodof time upon process color printing.

DISCLOSURE OF THE INVENTION

In order to solve the foregoing subject, the present invention providesa method of making an offset printing plate comprising a blank plateattaching step of winding a plate-shaped blank plate having a heatsensitive layer to which an image is formed thermally on a supportaround the outer circumferential surface of a cylinder with the heatsensitive layer being directed outward, thereby making the blank platerotatable integrally with the cylinder, and an image forming step ofirradiating a group of beams comprising a plurality of infrared laserbeams arranged in line to the blank plate on the outer circumferentialsurface of the cylinder based on an image forming signal while rotatingthe cylinder, thereby forming an image in accordance with the imageforming signal to the heat sensitive layer of the blank plate, whereinirradiation conditions for a plurality of infrared laser beamsconstituting the group of beams are set such that the temperature of theblank plate is uniform in a region in which an image is formed at onceby the group of beams in line in the image forming step.

According to this method, since the temperature of the blank plate ismade uniform in the region in which an image is formed at once by thegroup of beams in line, the temperature of the blank plate is madeuniform over the entire region in which the image is formed in onerotation of the cylinder. Accordingly, image formation by a uniform heatsensitive reaction is conducted for the entire surface of the heatsensitive layer of the blank plate, for example, by repeating themovement of the group of beams in line in the direction of therotational axis of the cylinder on every one rotation of the cylinder.This can outstandingly improve the image quality of the obtainedprinting plate.

The blank plate attaching step in the method according to the presentinvention preferably has a step of securing the top end of the blankplate to the circumferential surface of the cylinder by a clampmechanism, in which positioning is conducted by utilizing one side atthe top end of the blank plate upon securing by the clamp mechanism andthe blank plate is attached while keeping the positioned state.

According to this method, since the blank plate has been positioned byutilizing one side at the top end of the blank plate before the imageforming step, an image is formed at an accurate position relative to oneside as a reference of the blank plate in the image forming step. Thisenables to conduct positioning also in the process color printing by aconvenient operation and accurately.

After the image forming step in the method of the present invention, apost treating step of irradiating UV-rays at a wavelength of 200 to 400nm to the heat sensitive layer of the blank plate is preferablyconducted. In this method, printing quality such as printing resistanceof an image area can be improved outstandingly by conducting the posttreatment step of UV-ray irradiation.

The present invention further provides a method of making an offsetprinting plate, comprising a blank plate attaching step of winding aplate-shaped blank plate having a heat sensitive layer to which an imageis formed thermally on a support around the outer circumferentialsurface of a cylinder with the heat sensitive layer being directedoutward, thereby making the blank plate rotatable integrally with thecylinder, and an image forming step of irradiating infrared laser beamsto the blank plate on the outer circumferential surface of the cylinderbased on an image forming signal while rotating the cylinder, therebyforming an image in accordance with the image forming signal to the heatsensitive layer of the blank plate, wherein a post treating step ofirradiating UV-rays at a wavelength of 200 to 400 nm to the heatsensitive layer of the blank plate is conducted after the image formingstep.

According to this method, printing quality such as printing resistanceof an image area is outstandingly improved by applying the post treatingstep of UV ray irradiation.

In a case where the heat sensitive layer contains microcapsulescontaining an oleophilic ingredient in the inside and thermallydestroyed, hydrophilic binder polymer having functional groups capableof three dimensional cross linking and functional groups capable ofreacting with the oleophilic ingredient, and photoreaction initiator forinitiating the three dimensional cross-linking reaction of thehydrophilic binder polymer as described in Japanese Published UnexaminedPatent Application Hei 7-1849, the hydrophilic binder polymer can bethree dimensionally cross linked by the post treating step. This canmodify the surface of the blank plate just after the image forming stepto remarkably improve the printing quality such as ink receptibility andtransferability, reproducibility of fine lines or mesh dots, or printingresistance.

Further, the present invention provides an apparatus for making anoffset printing plate, comprising a cylinder having a rotationalmechanism, a blank plate attaching mechanism for winding and securing aplate-shaped heat sensitive type blank plate (having a heat sensitivelayer on a support) to the outer circumferential surface of thecylinder, a cassette for keeping a plurality of blank plates, a blankplate supply mechanism of taking out blank plates from the cassette anddirecting them to the cylinder, a laser generation device for generatinga plurality of infrared laser beams in line, an irradiation conditionsetting device for setting irradiation condition (intensity orirradiation time) on each of infrared laser beams based on an imageforming signal and the position in the line, a laser irradiation head(hereinafter also referred to as “optical head”) having an opticalsystem for focusing a plurality of laser beams irradiated from the lasergeneration device to the blank plate wound around the outercircumferential surface of the cylinder, and a head moving mechanism forlinearly moving the laser irradiation head along a line opposing inparallel with the rotational axis of the cylinder at a position spacedapart by a predetermined distance from the cylinder.

The group of the laser beams in line to be generated from the lasergeneration device may be laser beams disposed only by one in the lateraldirection of the line, or it may be disposed in plurality. Accordingly,the laser generation device can be obtained, for example, by providing aplurality of optical fibers coupled to semiconductor lasers andarranging each of the optical fibers in one direction at an equaldistance, or arranging them both in the longitudinal direction and thelateral direction of the line each by a predetermined number at an equaldistance.

In this plate making apparatus, the plate-shaped heat sensitive blankplate is wound and secured to the outer circumferential surface of thecylinder with a heat sensitive layer being directed outward, thecylinder is rotated in this state and the laser generation device isoperated, and a laser beam is irradiated over the entire surface of theblank plate of the outer circumferential surface of the cylinder byrepeating movement of the irradiation head each by a predeterminedamount by the head moving mechanism, on every one rotation of thecylinder for example. Further, by the setting of the irradiationcondition setting device, an image in accordance with the image formingsignal is formed to the heat sensitive layer of the blank plate.

Particularly, when the irradiation condition of each of the infraredlaser beams is set, for example, such that the irradiation energy is lowfor the laser beam at the center of the line and the irradiation energyis high for the laser beam on the ends of the line based on the positionin the line, the temperature of the blank plate can be made uniformwithin a region in which an image is formed at once by a group of laserbeams arranged in line.

In the plate making apparatus according to the present invention,preferably the blank plate supply mechanism has a conveying device forconveying a blank plate from the laterally direction to the cylinder,the blank plate attaching mechanism has a clamp mechanism for securingthe top end of the blank plate conveyed by the conveying device to thecircumferential surface of the cylinder, and the clamp mechanism has apositioning surface for being touched by the top end face of the blankplate. With such a constitution, positioning can be conducted easily byutilizing one side at the top end of the blank plate upon securing thetop end of the blank plate by the clamp mechanism.

The plate making apparatus according to the present invention preferablyhas a UV-ray irradiation device for irradiating UV-rays at a wavelengthof 200 to 400 nm to the heat sensitive layer of the blank plate and ablank plate moving mechanism for detaching the blank plate from thecylinder and directing the same to the UV-ray irradiation device.

Furthermore, the present invention provides an apparatus for making anoffset printing plate, comprising a cylinder of a structure capable ofwinding and securing a plate-shaped blank plate to the outercircumferential surface thereof, a rotational mechanism for thecylinder, a laser generation device for generating a laser beam in aninfrared region based on an image forming signal, a laser irradiationhead having an optical system for focusing the laser beam from the lasergeneration device to the blank plate on the outer circumferentialsurface of the cylinder, a head moving mechanism for moving theirradiation head along a line opposing in parallel with the rotationalaxis of the cylinder at a position spaced apart by a predetermineddistance from the cylinder, a UV-ray irradiation device for irradiatingUV-rays at a wavelength of 200 to 400 nm to the heat sensitive layer ofthe blank plate and a blank plate moving mechanism for detaching theblank plate from the cylinder and directing the same to the UV-rayirradiation device.

According to this apparatus, the laser beam is irradiated over theentire surface of the blank plate on the outer circumferential surfaceof the cylinder after winding and securing the plate-shaped blank platehaving a heat sensitive layer on a support to the outer circumferentialsurface of the cylinder with the heat sensitive layer being directedoutward, and by rotating the cylinder in this state and operating thelaser generation device, and repeating movement of the irradiation headby a predetermined amount by the head moving mechanism, on one rotationof the cylinder, for example. This can form an image in accordance withthe image forming signal to the heat sensitive layer of the blank plate.Subsequently, the blank plate is detached by the blank plate movingmechanism from the cylinder and directed to the UV-ray irradiationdevice, and the heat sensitive layer thereof is irradiated with UV-raysat a wavelength of 200 to 400 nm.

In the plate making apparatus according to the present invention, theapparatus having the UV-ray irradiation device and the blank platemoving mechanism is suitable to a case in which the heat sensitive layercontains microcapsules containing an oleophilic ingredient in the insideand destroyed thermally, hydrophilic binder polymer having functionalgroups capable of three dimensional cross linking and functional groupscapable of reacting with the oleophilic ingredient, and photoreactioninitiator for initiating three dimensional cross linking reaction of thehydrophilic binder polymer. Further, the print making apparatuspreferably has a blank plate attaching mechanism of winding aplate-shaped blank plate to the outer circumferential surface of thecylinder and capable of rotating the same integrally therewith.

As a light source of the post treating device, a fluorescent lamp havingwavelength peaks in emission wavelength regions of 300 to 400 nm and 360to 370 nm (chemical lamp) or a fluorescent lamp having wavelength peaksin emission wavelength regions of 200 to 300 nm and 250 to 255 nm(sterilizing lamp) can be used. Further, the chemical lamp and thesterilizing lamp can be used together.

As the light source for the post-treating device, a high pressuremercury lamp having an emission wavelength region of 200 to 500 nm,superhigh pressure mercury lamp, or metal halide lamp can be used.

When the high pressure mercury lamp, superhigh pressure mercury lamp, ormetal halide lamp is used as the light source for the post-treatingdevice, a cold mirror or a heat ray absorption glass is preferablydisposed each alone or in combination. Further, if a blank plate isdeteriorated by UV-rays in a specific wavelength region, a filter forcutting UV-rays in such a wavelength region is preferably disposed.

When the high pressure mercury lamp, superhigh pressure mercury lamp ormetal halide lamp is used as the light source for the post-treatingdevice, the light source is preferably inserted in a water-cooled bluefilter jacket tube for cutting a wavelength at 450 nm or higher.

As the light source for the post-treating device, a UV ray laser havingan oscillation wavelength in an ultraviolet region such as an He-Cd mayalso be used.

Further, the post treating device is preferably constituted such thatUV-rays can be irradiated to the blank plate in a state wound around thecylinder without attaching the blank plate from the cylinder. Theconstitution for this purpose can include, for example, an arrangementof disposing the light source to the periphery of the cylinder ortransmission of UV-rays through optical fibers from the UV-raygeneration device to the outer circumference of the cylinder.

In a case of using optical fibers for the irradiation of UV-rays, it ispreferably constituted such that the top ends of the optical fibers forirradiation of UV-rays are disposed together on a moving stage forattaching an optical head that irradiates infrared beams for imageformation, the top ends of the optical fibers for irradiation of UV-raysare arranged at a position behind the optical head along the movingdirection of the stage upon forming the image, so that UV-rays can beirradiated to the surface of the blank plate simultaneously with imageformation by the infrared beams.

In the print making apparatus according to the present invention, theimage forming width of the laser beam by the optical head is determineddepending on the number of the laser beams and resolution of the imageformed to the blank plate, and the moving amount of the optical head isset in accordance with the image forming width.

Further, it is preferably constituted such that the size of the blankplate in circumferential direction is made smaller than the cylindercircumference (up to about 70 to 80% of the cylinder circumference), toprovide a marginal portion not mounted with the blank plate to the outercircumferential surface of the cylinder and the optical head is movedwhile it is opposed to the marginal portion.

As an image forming signal used for a CTP system, a digital imagerecording signal (bit map data) formed, for example, by applying an RIP(Raster Image Processor) process to an image data edited by a DTP (DeskTop Publishing) of a computer or an electronic composing machine isutilized.

The bit map data is, for example, compressed optionally in an RIPsection, received by a control computer and stored in a main memory, andthe compressed bit map data is optionally restored into an originaldata, and sent to a line memory of electronic control device. Further, arotary encoder is disposed on the axis of the cylinder and the data ofthe rotational angle measured by the rotary encoder are sequentiallytaken into the electronic control device.

Then, the coordinate for the start position of the laser irradiation tothe blank plate wound around the cylinder is calculated on real timeand, at the same time, a coordinate for the completion position of thelaser irradiation is calculated from an optimal irradiation time onevery laser within a range of a maximum laser irradiation time inducedfrom the inter-pixel pitch determined depending on a desired resolutionand the rotational circumferential speed of the cylinder. Then, thecoordinate for the start position of the laser irradiation and thecoordinate for the completion position of irradiation are superimposedon the image signal of the line memory to prepare a control signal andthe laser generation device is controlled by the control signal.

Further, an infrared ray intensity measuring sensor is disposed on anoptical path of the semiconductor laser beam to sample a laser intensityupon actuation of the plate making apparatus or at an appropriate timingand the laser intensity data is taken into the control computer.Further, the data is calculated in comparison with a previouslyregistered set value on each lasers and a driving input current for thesemiconductor laser is controlled in accordance with the input currentand the output intensity characteristic of the semiconductor laser tokeep the intensity of each laser beam irradiated to the blank platealways at a predetermined value.

Alternatively, a photosensor is disposed near the opposing side of thesemiconductor oscillator on the side of the emitter (the laser beamemitting port) and the laser intensity is sampled on real time uponoscillation of the semiconductor laser. Then, the intensity data istaken into the control computer and the same calculation as describedabove is conducted by an automatic calculation function to control theinput current for driving the semiconductor laser to keep the intensityof each laser beam irradiated to the blank plate always at apredetermined value.

Since the focal position of the laser beam is displaced subtly from thesurface of the blank plate on the outer circumferential surface of thecylinder depending on the difference of the thickness of the blankplate, circularity of the outer surface of the cylinder, deflection ofthe cylinder during rotation, or thermal expansion or thermal shrinkageof the cylinder or the like caused by the change of the atmospherictemperature in the plate making apparatus, the optical system preferablycomprises an automatic focus correction mechanism adapted to move anobjective lens in a direction vertical to the blank plate to alwaysfocus the laser beam at the surface of the blank plate.

The infrared laser constituting the laser generation device ispreferably a semiconductor laser emitting an infrared rays at anemission wavelength of 750 to 880 nm and at the maximum power of 100 mWto 20 W, and the semiconductor laser is preferably used under PWM (PulseWidth Modulation) by directly controlling the input current at amodulation speed within a range from 0.1 to 10 Mbit/sec.

The laser beam from the laser generation device has preferably aconstitution to be transmitted through optical fibers to the opticalhead.

The optical system is preferably incorporated with a zoom mechanismcapable of automatically changing the optical magnification factor inaccordance with a desired resolution. Further, the optical system ispreferably constituted such that the beam spot diameter focused to theblank plate on the outer circumferential surface of the cylinder is from5 to 50 μm.

An air blow and a vacuum suction mechanism are preferably disposed nearthe top end of the optical head with an aim of removing mists evaporatedand scattered from the surface of the blank plate by thermal reaction inthe course of image formation by the irradiation of the laser beams tothe blank plate wound around the cylinder.

The plate making apparatus is preferably constituted to blow cleaningair into the plate making apparatus to keep the inside of the apparatusin a pressurized state by the provision of the air blower and the airfilter.

Further, the rotational speed of the cylinder is preferably from 50 to3000 rpm.

BRIEF EXPLANATION OF DRAWINGS

FIG. 1 is a schematic side elevational view showing a print makingapparatus corresponding to a first embodiment according to the presentinvention.

FIG. 2 is a schematic plan view of the plate making apparatus shown inFIG. 1.

FIG. 3 is a schematic perspective view showing a laser generation deviceconstituting the plate making apparatus shown in FIG. 1.

FIG. 4 is a schematic cross sectional view showing an optical headconstituting the plate making apparatus shown in FIG. 1.

FIG. 5 is a schematic side elevational view showing a blank plate supplymechanism and a blank plate attaching mechanism constituting the platemaking apparatus shown in FIG. 1.

FIG. 6 is a schematic perspective view showing a plate attachingmechanism constituting the plate making apparatus shown in FIG. 1.

FIG. 7 is a schematic side elevational view showing a plate makingapparatus corresponding to a second embodiment according to the presentinvention.

FIG. 8 is a schematic plan view of the plate making apparatus shown inFIG. 7.

FIG. 9 is a schematic side elevational view showing the constitution ofthe apparatus conducting a post treating step by irradiation of UV-raysin the plate making apparatus shown in FIG. 7.

BEST MODE FOR PRACTICING THE INVENTION First Embodiment

A first embodiment of the plate making apparatus according to thepresent invention is to be explained with reference to FIGS. 1 to 6.

As shown in FIG. 1 and FIG. 2, a plate making apparatus 100 comprises ahollow cylinder 131 having a rotational mechanism, a cassette 121 forkeeping a plurality of blank plates 400, a blank plate supply mechanism120, a laser generation device 140, an optical head (laser irradiationhead) 150, a linear stage (head moving mechanism) 160, a plate dischargemechanism 170, a plate discharge conveyor 180, a plate receiving tray19, a control computer 200, an electronic control device (irradiationcondition setting device) 210, and an RIP server 220 (computer connectedto a network for exclusively conducting RIP process). Further, the platemaking apparatus 100 has a blank plate attaching mechanism 130 shown inFIG. 5 and FIG. 6. Reference numeral 900 in FIG. 1 indicates a vibrationproof rubber.

The blank plate 400 is a heat sensitive type offset blank plate and theblank plate used herein comprises a hydrophilic layer as a heatsensitive layer formed on a support made of a thin aluminum sheet, thehydrophilic sensitive layer comprising a material that containsmicrocapsules containing an oleophilic ingredient in the inside anddestroyed thermally, hydrophilic binder polymer having functional groupscapable of three-dimensional crosslinking and functional groups capableof reacting with the oleophilic ingredient, and photoreaction initiatorfor initiating three-dimensional crosslinking reaction of thehydrophilic binder polymer. Such a blank plate is formed, for example,by a method described in Japanese Patent Unexamined Publication No.7-1849.

The cassette 121 has a structure capable of keeping about 100 sheets ofblank plates in stack with the heat sensitive layer being faced upward,and supplement of the blank plate is informed by a photosensor fordetecting the absence or presence of the blank plate 400.

The blank plate supply mechanism 120, as shown in FIG. 5, has a vacuumsuction pad 122 for sucking under vacuum the upper surface of the blankplate 400 to take out the blank plate 400 from the cassette 121, and agroup of rolls 123 for transporting the blank plate 400 toward thehollow cylinder 131 while receiving the lower surface of the blank plate400 and preventing sagging of the lower end thereof. Thus, the blankplate 400 is conveyed to the hollow cylinder 131 from the lateraldirection.

The blank plate attaching mechanism 130, as shown in FIGS. 5 and 6, hasa top end clamp mechanism 300, a rear end clamp mechanism 301, a squeezeroll 325 and a vacuum suction mechanism 320.

The top end clamp mechanism 300 is attached at a predetermined positionof the hollow cylinder 131 for seizing the top end of the blank plate400 and, has a seizing surface opposing to the circumferential surfaceof the hollow cylinder 131 and a positioning surface 300A opposing tothe top end surface of the blank plate 400 being conveyed toward thehollow cylinder 131. The rear end clamp mechanism 301 is attached at apredetermined position of the hollow cylinder 131 for seizing the rearend of the blank plate 400 and the structure thereof is identical withthat of the top end of the clamp mechanism 300.

Accordingly, the top end of the blank plate 400 being conveyed by theblank plate supply mechanism 120 in the lateral direction to the hollowcylinder 131 is inserted in a gap (several millimeters) between the topend mechanism 300 and the cylinder surface, and touched against apositioning surface 300A with a weak force. Since positioning is thusconducted by utilizing one side at the top end of the blank plate 400,image positioning for four plates in the subsequent process colorprinting step can be conducted easily.

The blank plate supply mechanism 120 has a mechanism of finelycorrecting the conveying speed of the blank plate 400 such that the topend surface of the blank plate 400 touches the positioning surface 300Aof the top end clamp mechanism 300 uniformly over the entire surfacewithout causing twisting or the like at the top end of the blank plate400.

After the positioning, the opposing surface of the top end clampmechanism 300 to the circumferential surface of the cylinder movestoward the circumferential surface of the cylinder 131, thereby the topend of the blank plate 400 is put and held between the top end clampmechanism 300 and the circumferential surface of the hollow cylinder 131while being kept in the positioned state. In this state, the hollowcylinder 131 is rotated and, at the same time, the squeeze roll 325 ispushed against the blank plate 400. Thus, the blank plate 400 is woundaround the hollow cylinder 131 and the rear end thereof is seized by therear end clamp mechanism 301. In this way, the blank plate 400 conveyedfrom the blank plate supply mechanism 120 is wound around thecircumferential surface of the hollow cylinder 131 while being kept inthe positioned state.

The vacuum suction mechanism 320 is used for firmly holding the blankplate 400 wound around the circumferential surface of the hollowcylinder 131 to the hollow cylinder 131, so that the attaching positiondoes not change even if the hollow cylinder 131 is rotated at a highspeed.

As shown in FIG. 6, the vacuum suction mechanism 320 comprises vacuumsuction holes 321 (fine through holes of about 1 to 3 mm diameter)formed to the outer circumferential surface of the hollow cylinder 131,an evacuation/air supply source 323 for discharging air from a cavity inthe hollow cylinder 131, and a pipeline 322 connecting the inside of thehollow cylinder 131 with the evacuation/air supply source 323. Thepipeline 322 is disposed passing through the inside of the shaft 133 andthe end thereof on the side of the hollow cylinder 131 is disposed inthe cavity of the hollow cylinder 131. Further, the shaft 133 and thepipeline 322 are connected with a rotatable rotary joint 324.

Accordingly, after winding the blank plate 400 to the hollow cylinder131 as described above, and then evacuating air in the hollow cylinder131 by the evacuation/air supply source 323, thereby air in the gapbetween the hollow cylinder 131 and the blank plate 400 is compulsorilydischarged through the vacuum suction holes 321. As a result, the blankplate 400 is firmly secured by vacuum suction.

The hollow cylinder 131 is installed horizontally on a rack base 110.The rotational mechanism of the hollow cylinder 131 comprises shafts 132and 133 protruded from both ends, bearings 134 for rotatably supportingthe shafts 132 and 133, a rotation motor 136 connected to the end of theshaft 132 with a coupling 135, and a rotary encoder 137 disposed to theend of the shaft 133 for measuring the rotational angle of the hollowcylinder 131.

The rotation motor 136 having a power of rotating the hollow cylinder131 at a rotational speed of 50 to 3000 rpm is used. When the blankplate has a large size, the outer diameter of the hollow cylinder 131is, for example, from 250 to 500 mm. When highly fine image dataexceeding 1000 dpi (dot/inch) are formed as an image by using such alarge hollow cylinder 131, it is practically preferred to keep therotational speed of the hollow cylinder 131 to about 1000 rpm or lowerin view of the restriction for the performance of a general opticalrotary encoder measuring system. A high performance optical rotaryencoder having high resolution is easily available from “HEIDENHAIN Co.”or “Canon Co.”.

The laser generation device 140 is used for generating a laser beam 800in an infrared region to be irradiated to the blank plate 400. As shownin FIG. 3, it comprises a plurality of semiconductor layers 141, a heatsink base 142 having cooling Peltier devices mounted thereon, a laserdriving device 143, and a fiber bundle 144. The plurality ofsemiconductor lasers 141 are fiber-coupled and disposed on the heat sinkbase 142.

As the semiconductor laser 141, those generating infrared laser at anoscillation wavelength of 750 to 880 nm are used and it is preferred toselect those having an optimal oscillation wavelength in accordance withthe absorption spectrum of an infrared absorbent added to the heatsensitive layer of the blank plate 400. Further, it is most preferred touse a semiconductor layer having an oscillation wavelength at 810 to 850nm in view of the overall performance as the device such as size, costand working life.

In a case of forming a highly fine image with resolution exceeding 1000dpi, the core diameter of the optical fiber coupled to the semiconductorlaser 141 is preferably 100 μm or less, and numeral aperture (NA) isgenerally from 0.12 to 0.15. Such a fiber-coupled semiconductor laser isavailable easily from “SDL Co.” or “OPTOPOWER Co.”.

As the fiber bundle 144, those comprising bundled fibers having the sameshape and function as the optical fibers used for the fiber-coupledsemiconductor laser 141 are used. Each of the optical fibers of thefiber bundle 144 is connected with the semiconductor laser 141 by aconnector or fusion splicing.

In the sheath at the top end of the fiber bundle 144, optical fibers arearranged laterally each at an equal distance with the pitch of severalhundreds μm and aligned and fixed such that laser beam from each of theoptical fibers is in parallel with each other. Thus, a group of laserbeams arranged in line are generated from the sheath at the top end ofthe fiber bundle 144.

Further, if the length of the fiber bundle 144 is as long severalmeters, it is preferred to insert the fiber bundle 144 into a flexiblepipe made of plastic or metal for protection.

If the semiconductor laser 141 is a laser that generates an outputenergy of about 1 W, a voltage at about 2-3 V is applied as a DC powersource from the laser driving device 143 to the semiconductor laser 141.It is preferred that a current of about 500 to 2000 mA at the maximum issupplied to the semiconductor laser 141 upon image formation, while itis preferred to supply a bias current of 20 to 100 mA which is a currentgiving no thermal effects on the surface of the blank plate 400 when theimage is not formed such that the semiconductor laser 141instantaneously reaches the maximum power intensity.

The top end sheath of the fiber bundle 144 is held, as shown in FIG. 4,by a fiber bundle securing portion 151 of an optical head 150.

The optical head 150 comprises a lens cylinder 152, a group ofcondensing lenses 153, a prism 154, a group of zoom lenses 155, a zoommechanism 156, a zoom motor 157, a group of objective lenses 158, anobjective lens-actuator 159 and an astigmatism sensing mechanism 500.

The infrared laser beam irradiated from the semiconductor laser 141 istransmitted through the optical fibers and, finally, emitted from thefinal end of the sheath of the fiber bundle 144 as the group of laserbeams arranged in line to the outside. The group of condensing lenses153 condense the laser beams into parallel light, and the infrared laserbeams converted into the parallel light are focused on the surface ofthe blank plate 400 wound around the hollow cylinder 131 into a beamspot diameter of several to several tens μm through the prism 154, thegroup of zoom lenses 155, and the group of objective lens 158.

The beam spot diameter to be focused on the surface of the blank plate400 can be optionally determined by varying the optical reduction factorof the group of zoom lens 155 and the group of objective lens 158.Practically, the lenses having a maximum reduction factor of about 5 areselected with the reason, for example, of intending to ensure a distanceof (working distance) several μm or more from the top end of the opticalhead 150 to the surface of the blank plate 400 and intending to minimizethe intensity loss of the laser beams without enlarging the size of theoptical system such as the lens or the lens cylinder 152 extremely.

For this purpose, if the fiber core diameter used for the fiber bundle144 is 50 μm, a beam spot diameter of about 10 μm at the minimum can beobtained on the surface of the blank plate 400. A further smaller beamspot diameter can of course be obtained by making the fiber corediameter of the fiber bundle 144 smaller. Further, while smaller beamspot diameter can also be obtained by choosing a lens with a furthermaximum reduction factor, the intensity loss of the laser beam isincreased.

Further, the zoom lens group 155 is adapted to change the relativeposition in accordance with the movement of the zoom mechanism 156.Since the zoom mechanism 156 advances or retracts and the relativeposition in the zoom lens group 155 is also changed together by therotation of the zoom motor 157 that is gear-coupled with the zoommechanism 156, the optical reduction factor is changed in accordancetherewith. Then, if the zoom lens is chosen such that the zoom factorcan be varied within range from 1 to 5 times, the beam spot diameterfocused on the surface of the blank plate 400 can be changed opticallywithin a range from 10 to 50 μm.

The astigmatism sensing mechanism 500 comprises a visible lightsemiconductor laser 501 having a wavelength region of 600 to 700 nm anda maximum power energy of about several tens mW, a beam shapingmechanism 502, a prism group 503, an automatic power control mechanism504 and a 4-divisional photodetector 505. A visible light laser beamirradiated from the visible light semiconductor laser 501 is shaped bythe beam shaping mechanism 502 into parallel light and separatedpartially at the prism 503. The separated beam is detected by thephotodiode of the automatic power control mechanism 504. The currentsupplied to the visible light semiconductor laser 501 is controlled bythe output signal of the photodiode to keep the output power of thelaser constant.

The visible light laser beam other than the beam transmitting the prism503 is reflected at a diagonal plane of the prism 154, superimposed withthe image forming infrared laser beam 800 and entered to the blank plate400. Most of the visible light laser beam is reflected on the surface ofthe blank plate 400 and entered again in the plasmas 154 and 503 andreflected. The reflected light is given with astigmatism by acylindrical lens on the optical path and finally fed back to the4-divisional photodetector 505.

In this mechanism, output signals of the 4-divisional photodetector 505are added diagonally to each other and further subtracted diagonallyfrom each other, and these values are inputted as focus error signals toa focus-servo control circuit and an objective lens-actuator 159 isoperated by the output signal from the focus-servo control circuit. Bythe mechanism, the objective lens group 158 suspended by a leaf springfrom the objective lens-actuator 159 moves forward and backward. Thus,the image forming infrared laser beam 800 is focused together with thevisible light laser beam on the surface of the blank plate.

On the other hand, the optical head 150 is placed on the linear stage160 as a movable support means, and can be moved linearly by the linearstage 160 in the longitudinal direction of the axis of the hollowcylinder 131. The linear stage 160 comprises a linear motor guide 161disposed in parallel with the hollow cylinder 131, a linear motor 162, alinear scale 163 and a support table 164 used for the optical headconnected with the linear motor 162.

Image formation by the optical head 150 (irradiation of the laser beam)is conducted over the entire surface of the blank plate 400 by themovement of the linear stage 160 having the optical head mounted thereonand the rotation of the hollow cylinder 131. That is, image formationfrom the optical head 150 to the blank plate 400 is conducted for apredetermined width in the direction of the cylinder axis during onerotation of the hollow cylinder 131, and the optical head 150 moves by apredetermined amount in the direction of the cylinder axis on every onerotation of the hollow cylinder 131. The process is repeated in entireaxial direction of the cylinder.

Then, the size of the blank plate 400 in the circumferential directionof the hollow cylinder 131 is made smaller than the circumference of thehollow cylinder 131 (up to about 70 to 80% of the circumference) toprovide a marginal portion where the blank plate is not attached to theouter circumferential surface of the hollow cylinder 131. Then, theoperation of the linear stage 160 is controlled such that the opticalhead 150 is not moved while the optical head 150 opposes to the blankplate attaching surface of the hollow cylinder 131, and the optical head150 is moved by a predetermined amount in the direction of therotational axis of the hollow cylinder 131 while the optical head 150 isopposed to the marginal portion of the hollow cylinder 131.

Thus, when an image is formed to the entire surface of the blank plate400, it is no more necessary to stop the rotation of the hollow cylinderor form the image once per two rotations of the hollow cylinder 131(image is formed during first rotation and the linear stage is movedduring the succeeding rotation), so that the image can be formedefficiently over the entire surface of the blank plate 400 with noadditional useless rotation.

The moving amount of the optical head 150 is defined as a distanceobtained by multiplying the beam pitch corresponding to resolution ofthe image data to be formed to an image by the number of laser beams.

An RIP server 220 receives image data made by DTP or an electroniccomposing machine by a communication protocol such as TCP/IP or AppleTalk by way of a network line (Ethernet, etc.) and makes bit map data byapplying RIP process to the received image data. Subsequently, the bitmat data is compressed by an algorithm such as a run length method todecrease the capacity of the bit map data.

The control computer 200 receives the compressed bit map data from theRIP server 220 by way of the interface line, (for example, SCSI) andstores the data in the main memory (RAM) in the control computer 200.The control computer 200 properly defreezes the compressed bit mat datastored in the main memory and restores the data into the original bitmap data and then transfers the restored bit map data by way of acontrol bus (Compact PCI or VME bus) to the line memory of theelectronic control device 210.

The electronic control device 210 has two sets of line memory offunctions referred to as A bank/B bank and forms an image with the bitmap data contained in one of the line memories (A bank) whiletransferring the bit map data for the next line to another empty linememory (B bank). It is adapted to complete transferring of the bit matdata in parallel while forming an image within a period for one rotationof the hollow cylinder 131 by alternately switching image formation andrelocation.

Further, the electronic control device 210 has a receiving counter forthe data of rotational angle sent from the rotary encoder 137 andcalculates the basic number of pulses between pixels based on the outerdiameter of the blank plate 400, resolution angle per one pulse from therotary encoder 137 and setting resolution of the image. Further, theposition for starting image formation to the blank plate 400 iscalculated based on the rotational position information of the hollowcylinder 131 formed on real time in accordance with rotation of thehollow cylinder 131, to determine the position for completing imageformation on every laser based on the rotational circumferential speedof the hollow cylinder 131 and the laser irradiation time previouslydetermined individually on every laser.

Then, the electronic control device 210 superimposes the thus determinedposition for completing image formation on every laser and a logicsignal of the bit map data, and outputs the superimposed control signalto the laser driving device 143 of the laser generation device 140.Thus, the laser driving device 143 controls the image formation time onvery laser independently.

In this case, the set value for the irradiation time for each of thelasers is previously calculated based on the material and the thicknessof the heat sensitive layer of the blank plate 400 to be used and thebeam position at which the group of laser beams arranged in line areemitted finally. With respect to the beam position, the irradiation timeis set shorter at the center of the line, while the irradiation time isset longer toward the ends of the line. This can make the temperature ofthe blank plate 400 uniform within a region in which an image is formedat once by a group of laser beams arranged in line.

Accordingly, in the plate making apparatus, the temperature of the blankplate 400 is made uniform over the entire region in which the image isformed in one rotation of the hollow cylinder 131 and the group of beams800 arranged in line are moved repeated in the direction of the axis ofrotation on every one rotation of the hollow cylinder 131, so that theimage is formed by uniform heat sensitive reaction over the entiresurface of the heat sensitive layer of the blank plate 400. This canremarkably improve the image quality of the obtained printing plate.

Further, the plate making apparatus 100 has an infrared intensity sensor801 having a photo-receiving surface at the focusing position of theimage-forming infrared laser beam 800 beside the hollow cylinder 131, soas to move the linear stage 160 to a position at which the image forminginfrared resin beam 800 is detected by the infrared intensity sensor 801upon actuation of the plate making apparatus or at an appropriatetiming.

In this constitution, one laser is turned on by the laser driving device143 for several seconds, the measured intensity data is taken into thecontrol computer 200 to control the laser driving current of the lasergeneration device 140, and the laser beam is irradiated at apredetermined laser intensity to the blank plate 400. Then, by repeatingthe process successively for the number of the laser beams, the laserintensity is set on every laser independently.

Further, it may be adapted such that the window of the semiconductorlaser 141 opposing to an oscillator emitter window is made as ahalf-mirror structure, a portion of the laser beam generated in theoscillator is taken out and detected by the photodiode to control thelaser intensity like that in the means described above.

Further, a plate discharge mechanism 170 is disposed above the hollowcylinder 131 of the plate making apparatus 100. A vacuum suction pad isdisposed to the plate discharge mechanism 170, and the blank plate 400after completing the image formation is sucked under vacuum by thevacuum suction pad, detached out of the hollow cylinder 131 andtransported to the plate discharge conveyor 180. The blank plate 400transported to the plate discharge conveyor 180 is received by the platereceiving tray 19.

Second Embodiment

A second embodiment of the plate making apparatus according to thepresent invention is to be explained with reference to FIGS. 7 to 9. Ascan be seen from comparison between FIG. 1 and FIG. 7 and comparisonbetween FIG. 2 and FIG. 8, the plate making apparatus 100 is differentfrom the first embodiment, in that a UV-ray irradiation device 190 forirradiating UV-rays to a blank plate transported to the plate dischargeconveyor 180 is disposed but is identical with the first embodiment inother constitutions.

As shown in FIG. 9, a blank plate 410 on the plate discharge conveyor180 is put to a post treatment by irradiation of UV-rays from the UV-rayirradiation device 190 along with movement of the plate dischargeconveyor 180. By the post treatment, the printing resistance and theprinting quality for the obtained image portion of the plate areimproved remarkably.

A metal hydride lamp is used for the lamp 192 of the UV-ray irradiationdevice 190 and an inverter power source is used as a control powersource for the metal halide lamp, and the lamp intensity is optionallyvariable within a range from 25 to 100%. Further, the lamp is air-cooledby a air cooling exhaust blower 195 and an exhaust duct 194. Further,the lamp 192 is attached to a housing 191 capable of rotating by 180°and an aluminum reflection plate 193 is disposed at a position of thehousing 191 for the back of the lamp 192.

In this embodiment, since a long metal halide lamp can not be turned oninstantaneously, it is lighted up in a stand-by state with a weak lampintensity of about 25%, and a portion between the lamp 192 and a platedischarge conveyor 180 is shielded by the housing 191 so as not to leakUV-rays onto the plate discharge conveyor 180 by rotating the housingfor 180°.

Then, the blank plate 410 is detached from the hollow cylinder 131 by aplate take-out pad 170 and transported to a plate discharge conveyor 180and, at the same time, the plate discharge conveyor 180 is driven andthe housing 191 rotates by 180° to return to the position above the lamp192, and the power of the metal halide lamp 192 is increased to 100%lamp intensity.

Further, when the blank plate 410 passes below the UV-ray irradiationdevice 190, the housing 191 rotates by 180° and returns to the stand-byposition, and the power of the metal halide lamp 192 is lowered to aweak lamp intensity.

It is necessary to increase or decrease the amount of irradiation energyof UV-rays in accordance with the amount of irradiation energy ofUV-rays required for the blank plate, and this can be increased ordecreased by increasing or decreasing the lamp intensity of the metalhalide lamp 192 with inverter power supply. In addition to the above,since a speed variable mechanism is attached to the plate dischargeconveyor 180, the amount of irradiation energy of UV-rays can be easilyincreased or decreased by changing the speed of the plate dischargeconveyor 180.

In this embodiment, an air-cooled type metal halide lamp is used as thelamp 192 of the UV-ray irradiation device 190, and same effect can alsobe expected by using a high pressure mercury lamp, super-high pressuremercury lamp or a chemical lamp or sterilizing lamp providing that theemission wavelength is within a ultra-violet region of 200 to 400 nm.Accordingly, the lamp to be used can be selected properly depending onthe irradiation energy requiring for the blank plate.

Further, if temperature elevation is undesired for the blank plate, itis preferred to make the reflection plate with a cold mirror allowingonly the heat rays to permeate therethrough selectively instead of thealuminum reflection plate, or additionally dispose heat ray absorbingglass just below the lamp. For shielding heat rays more effectively, itis preferred to adopt a water-cooled type metal halide lamp of insertinga lamp in a water cooled blow filter jacket tube capable of cutting offvisible rays at 450 nm or higher or heat rays by nearly about 100%.

As shown in FIG. 4, it is preferred in the plate making apparatusaccording to the present invention to provide a vacuum suction mechanism600 between an optical lens head 150 and a hollow cylinder 131, toprevent mists that are evaporated and scattered by thermal reaction fromthe surface of the blank plate during image formation to the blank plate400 from depositing on the lens surface of the objective lens group 158.The vacuum suction mechanism 600 comprises a dust collecting hood 601, avacuum pump 603, a filter and an exhaust duct 602.

In this embodiment, the dust collecting hood 601 of the vacuum suctionmechanism 600 is disposed on the support table 164 and the vacuumsuction mechanism 600 is controlled to be moved together with the linearstage 160, for example, by the control computer 200.

Further, when the plate making apparatus according to the presentinvention is constituted as a tightly closed structure in which a coveris attached to the frame of the apparatus, clean air generated from aclean air supply mechanism 700 constituted with an air blower and an airfilter (refer to FIG. 1 and FIG. 7) is sent into the apparatus to keep apressurized state thereby keeping the inside of the apparatus clean,undesired effect of dusts or dirts in the atmosphere of the room can beeliminated, so that an offset printing plate of more excellent printingquality can be manufactured.

Industrial Applicability

As has been described above, the method of the present invention is aplate making method of forming an image to a heatsensitive type blankplate by an outer surface cylinder scanning system plate makingapparatus.

Then, according to the method of the present invention, since imageformation with a uniform heat sensitive reaction is conducted for theentire surface of the heat sensitive layer of the blank plate in animage forming step by a multi-channel system, the image quality of theobtained printing plate can be improved outstandingly. Further, byconducting positioning utilizing one side at the top end of the blankplate, accurate positioning for blank plates of four colors can beconducted conveniently in a short time upon process color printing usingthe thus obtained printing plate. Further, the printing quality of theobtained printing plate can be improved outstandingly by conducting thepost treating step.

In view of the above, according to the method of the present invention,a practical heat sensitive type offset printing plate can be obtained ata commercial level.

Further, according to the apparatus of the present invention, the methodof the present invention can be practiced with ease.

What is claimed is:
 1. A method of making an offset printing plate,comprising: a blank plate attaching step of winding a plate-shaped blankplate having a heat sensitive layer to which an image is formedthermally on a support around an outer circumferential surface of acylinder with the heat sensitive layer being directed outward, therebymaking the blank plate rotatable integrally with the cylinder; and animage forming step of irradiating a group of beams comprising aplurality of infrared laser beams arranged in line to the blank plate onthe outer circumferential surface of the cylinder based on an imageforming signal while rotating the cylinder, thereby forming an image inaccordance with the image forming signal to the heat sensitive layer ofthe blank plate; wherein, in the image forming step, the group of beamsin line are irradiated to a single area on the blank plate; and whereinirradiation conditions for the plurality of infrared laser beamscomprising the group of beams are determined individually such that thetemperature of the blank plate is made uniform within the single area inwhich the image is formed at once by the group of beams in line.
 2. Themethod of making an offset printing plate as defined in claim 1, whereinthe blank plate attaching step has a step of securing the top end of theblank plate to the circumferential surface of the cylinder by a clampmechanism, positioning is conducted by utilizing one side at the top endof the blank plate upon securing by the clamp mechanism and the blankplate is attached while being kept in the positioned state.
 3. Themethod of making an offset printing plate as defined in claim 1, whereina post treating step of irradiating UV-rays at a wavelength of 200 to400 nm to the heat sensitive layer of the blank plate is conducted afterthe image forming step.
 4. The method of making an offset printing plateas defined in claim 2, wherein a post treating step of irradiatingUV-rays at a wavelength of 200 to 400 nm to the heat sensitive layer ofthe blank plate is conducted after the image forming step.
 5. A methodof making an offset printing plate, comprising: a blank plate attachingstep of winding a plate-shaped blank plate having a heat sensitive layerto which an image is formed thermally on a support around an outercircumferential surface of a cylinder with the heat sensitive layerbeing directed outward, thereby making the blank plate rotatableintegrally with the cylinder; and an image forming step of irradiating agroup of beams comprising a plurality of infrared laser beams to theblank plate on the outer circumferential surface of the cylinder basedon an image forming signal while rotating the cylinder, thereby formingan image in accordance with the image forming signal to the heatsensitive layer of the blank plate; wherein, in the image forming step,the group of beams in line are irradiated to a single area on the blankplate; and wherein a post treating step of irradiating UV-rays at a wavelength of 200 to 400 nm to the heat sensitive layer of the blank plateis conducted after the image forming step.
 6. The method of making anoffset printing plate as defined in claim 3, wherein the heat sensitivelayer contains microcapsules containing an oleophilic agent in theinside and destroyed thermally, hydrophilic binder polymer havingfunctional groups capable of three-dimensional crosslinking andfunctional groups capable of reacting with the oleophilic ingredient,and photoreaction initiator for initiating the three-dimensionalcrosslinking reaction of the hydrophilic binder polymer, and thehydrophilic binder polymer is three-dimensionally crosslinked by thepost treating step.
 7. The method of making an offset printing plate asdefined in claim 4, wherein the heat sensitive layer containsmicrocapsules containing an oleophilic agent in the inside and destroyedthermally, hydrophilic binder polymer having functional groups capableof three-dimensional crosslinking and functional groups capable ofreacting with the oleophilic ingredient, and photoreaction initiator forinitiating the three-dimensional crosslinking reaction of thehydrophilic binder polymer, and the hydrophilic binder polymer isthree-dimensionally crosslinked by the post treating step.
 8. The methodof making an offset printing plate as defined in claim 5, wherein theheat sensitive layer contains microcapsules containing an oleophilicagent in the inside and destroyed thermally, hydrophilic binder polymerhaving functional groups capable of three-dimensional crosslinking andfunctional groups capable of reacting with the oleophilic ingredient,and photoreaction initiator for initiating the three-dimensionalcrosslinking reaction of the hydrophilic binder polymer, and thehydrophilic binder polymer is three-dimensionally crosslinked by thepost treating step.
 9. An apparatus for making an offset printing plate,comprising: a cylinder having a rotational mechanism, a blank plateattaching mechanism for winding and securing a plate-shaped heatsensitive type blank plate to the outer circumferential surface of thecylinder, a cassette for keeping a plurality of blank plates, a blankplate supply mechanism for taking out the blank plates from the cassetteand directing them to the cylinder, a laser generation device forgenerating a group of beams comprising a plurality of infrared laserbeams arranged in line, a laser irradiation head having an opticalsystem for focusing the group of beams comprising a plurality of laserbeams irradiated from the laser generation device to a single area onthe blank plate, an irradiation condition setting device for settingirradiation conditions on each of infrared laser beams based on an imageforming signal and the position in the line such that the temperature ofthe blank plate is made uniform within the single area in which an imageis formed at once by the group of beams in line, and a head movingmechanism for linearly moving the laser irradiation head along a lineopposing in parallel with the rotational axis of the cylinder at aposition spaced apart by a predetermined distance from the cylinder. 10.The apparatus for making an offset printing plate as defined in claim 9,wherein the blank plate supply mechanism comprises a conveying devicefor conveying the blank plate from the lateral direction to thecylinder, the blank plate attaching mechanism has a clamp mechanism forsecuring the top end of the blank plate conveyed by the conveying deviceto the circumferential surface of the cylinder, and the clamp mechanismhas a positioning surface for being touched by the top end face of theblank plate.
 11. The apparatus for making an offset printing plate asdefined in claim 10, wherein the apparatus comprises a UV-rayirradiation device for irradiating UV-rays at a wavelength of 200 to 400nm to the heat sensitive layer of the blank plate and a blank platemoving mechanism for detaching the blank plate from the cylinder anddirecting the same to the UV-ray irradiation device.
 12. An apparatusfor making an offset printing plate, comprising: a cylinder of astructure capable of winding and securing a plate-shaped blank platearound the outer circumferential surface, a rotational mechanism for thecylinder, a laser generation device for generating a group of beamscomprising a plurality of infrared laser beams arranged in line, a laserirradiation head having an optical system for focusing the group ofbeams comprising a plurality of laser beams irradiated from the lasergeneration device to a single area on the blank plate, an irradiationcondition setting device for setting irradiation conditions on each ofinfrared laser beams based on an image forming signal and the positionin the line such that the temperature of the blanket plate is madeuniform within the single area in which an image is formed at once bythe group of beams in line, a head moving mechanism for moving theirradiation head along with a line opposing in parallel with therotational axis of the cylinder at a position spaced apart by apredetermined distance from the cylinder, a UV-ray irradiation devicefor irradiating UV-rays at a wavelength of 200 to 400 nm to the heatsensitive layer of the blank plate, and a blank plate moving mechanismfor detaching the blank plate from the cylinder and the directing thesame to the UV-ray irradiation device.
 13. The method of making anoffset printing plate as defined in claim 11, wherein the heat sensitivelayer contains microcapsules containing an oleophilic agent in theinside and destroyed thermally, hydrophilic binder polymer havingfunctional groups capable of three-dimensional crosslinking andfunctional groups capable of reacting with the oleophilic ingredient,and photoreaction initiator for initiating the three-dimensionalcrosslinking reaction of the hydrophilic binder polymer, and thehydrophilic binder polymer is three-dimensionally crosslinked by thepost treating step.
 14. The method of making an offset printing plate asdefined in claim 12, wherein the heat sensitive layer containsmicrocapsules containing an oleophilic agent in the inside and destroyedthermally, hydrophilic binder polymer having functional groups capableof three-dimensional crosslinking and functional groups capable ofreacting with the oleophilic ingredient, and photoreaction initiator forinitiating the three-dimensional crosslinking reaction of thehydrophilic binder polymer, and the hydrophilic binder polymer isthree-dimensionally crosslinked by the post treating step.